Although durable enough to use on both interior and exterior surfaces (pretty much any surface will accept the coating), for best results, we recommend sticking to surfaces that do not have prolonged moisture contact or water submersion. Our official recommendation is to stick to the outer surfaces of the toilet, sink or bathtub, rather than inside the bowl where significant water submersion will occur. This is solely from our company's longevity guarantee.
That being said, we do know many, many, many customers who have used it successfully inside sinks, bathtubs and toilets. Surfaces with a lot of water exposure tend to degrade any coating applied, not to mention that they have a higher likelihood for rust and mold, which our coating does not protect against. But you'll need to make the best decision for yourself based on location, size and condition of the damage.
If you do use it on a surface with lengthy water exposure or submersion, the following will help give the best results:
First, prepare the surface by making sure it is clean of any debris (dust, rust, etc.). If there is any rust, you will definitely want to sand it away, and you may want to use a rust inhibitor on the bare metal. Vacuum and/or wash away any dust and debris, and thoroughly clean the area and make sure it is dry before painting. Next, paint the area using several thin coats, making sure they dry in between. (The paint will likely dry to the touch in about half an hour, but it's best to wait 2-4 hours between coats, for optimum results.)
Full curing of the paint usually takes about 2-4 weeks, so it will be best if you can avoid exposing it to water until then. If that's not possible, you can temporarily cover the area with a waterproof covering (for example, by taping plastic wrap over the area) while it's in use, and then uncover it when you're done, to continue to cure.